Non-plasmonic nanoantennas for surface enhanced spectroscopies with ultra-low heat conversion

Multiple Scattering-Enhanced Fluorescence Within Randomly Oriented Low-Index Polymer Nanofiber Sensors

Fluorescence enhancement technologies play a crucial role in biological and chemical sensors. Currently, effective fluorescence sensors primarily rely on noble metals and high-index dielectric nanostructures. While effective, they are plagued by optical losses and complex fabrication processes. In contrast, low-index material nanostructures offer significant advantages, including the absence of optical losses, ease of fabrication, and cost-effectiveness, but they face the challenge of weaker electric field enhancement. Here, we designed a low-index, randomly oriented polyvinyl acetate (PVAc) nanofiber sensor via scalable electrospinning, enabling multiple scattering within the disordered nanofibers and resulting in an impressive surface-enhanced fluorescence factor of 1170. This sensor achieves a detection limit for rhodamine 6G as low as 7.24 fM, outperforming the reported fluorescence biosensors. Further results of photoluminescence decay dynamics and random lasing validate the effectiveness of multiple scattering in enhancing fluorescence within the polymer nanofiber sensor. With its excellent performance and scalable production process, this randomly oriented, low-index polymer nanofiber sensor offers a promising new pathway for efficient surface-enhanced fluorescence based on multiple scattering. Furthermore, PVAc nanofibers can be extended to other low-index materials capable of forming randomly oriented nanostructures, offering significant potential for cost-effective, high-performance fluorescence sensor applications.

Surface-enhanced Raman spectroscopy: a half-century historical perspective

Surface-enhanced Raman spectroscopy (SERS) has evolved significantly over fifty years into a powerful analytical technique. This review aims to achieve five main goals. (1) Providing a comprehensive history of SERS's discovery, its experimental and theoretical foundations, its connections to advances in nanoscience and plasmonics, and highlighting collective contributions of key pioneers. (2) Classifying four pivotal phases from the view of innovative methodologies in the fifty-year progression: initial development (mid-1970s to mid-1980s), downturn (mid-1980s to mid-1990s), nano-driven transformation (mid-1990s to mid-2010s), and recent boom (mid-2010s onwards). (3) Illuminating the entire journey and framework of SERS and its family members such as tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and highlighting the trajectory. (4) Emphasizing the importance of innovative methods to overcome developmental bottlenecks, thereby expanding the material, morphology, and molecule generalities to leverage SERS as a versatile technique for broad applications. (5) Extracting the invaluable spirit of groundbreaking discovery and perseverant innovations from the pioneers and trailblazers. These key inspirations include proactively embracing and leveraging emerging scientific technologies, fostering interdisciplinary cooperation to transform the impossible into reality, and persistently searching to break bottlenecks even during low-tide periods, as luck is what happens when preparation meets opportunity.

Optical Metasurfaces for Biomedical Imaging and Sensing

Optical metasurfaces, arrays of nanostructures engineered to manipulate light, have emerged as a transformative technology in both research and industry due to their compact design and exceptional light control capabilities. Their strong light-matter interactions enable precise wavefront modulation, polarization control, and significant near-field enhancements. These unique properties have recently driven their application in biomedical fields. In particular, metasurfaces have led to breakthroughs in biomedical imaging technologies, such as achromatic imaging, phase imaging, and extended depth-of-focus imaging. They have also advanced cutting-edge biosensing technologies, featuring high-quality factor resonators and near-field enhancements. As the demand for device miniaturization and system integration increases, metasurfaces are expected to play a pivotal role in the development of next-generation biomedical devices. In this review, we explore the latest advancements in the use of metasurfaces for biomedical applications, with a particular focus on imaging and sensing. Additionally, we discuss future directions aimed at transforming the biomedical field by leveraging the full potential of metasurfaces to provide compact, high-performance solutions for a wide range of applications.

Advancing precision cancer immunotherapy drug development, administration, and response prediction with AI-enabled Raman spectroscopy

Molecular characterization of tumors is essential to identify predictive biomarkers that inform treatment decisions and improve precision immunotherapy development and administration. However, challenges such as the heterogeneity of tumors and patient responses, limited efficacy of current biomarkers, and the predominant reliance on single-omics data, have hindered advances in accurately predicting treatment outcomes. Standard therapy generally applies a "one size fits all" approach, which not only provides ineffective or limited responses, but also an increased risk of off-target toxicities and acceleration of resistance mechanisms or adverse effects. As the development of emerging multi- and spatial-omics platforms continues to evolve, an effective tumor assessment platform providing utility in a clinical setting should i) enable high-throughput and robust screening in a variety of biological matrices, ii) provide in-depth information resolved with single to subcellular precision, and iii) improve accessibility in economical point-of-care settings. In this perspective, we explore the application of label-free Raman spectroscopy as a tumor profiling tool for precision immunotherapy. We examine how Raman spectroscopy's non-invasive, label-free approach can deepen our understanding of intricate inter- and intra-cellular interactions within the tumor-immune microenvironment. Furthermore, we discuss the analytical advances in Raman spectroscopy, highlighting its evolution to be utilized as a single "Raman-omics" approach. Lastly, we highlight the translational potential of Raman for its integration in clinical practice for safe and precise patient-centric immunotherapy.

Local Strain Engineering of Two-Dimensional Transition Metal Dichalcogenides Towards Quantum Emitters

Two-dimensional transition metal dichalcogenides (2D TMDCs) have received considerable attention in local strain engineering due to their extraordinary mechanical flexibility, electonic structure, and optical properties. The strain-induced out-of-plane deformations in 2D TMDCs lead to diverse excitonic behaviors and versatile modulations in optical properties, paving the way for the development of advanced quantum technologies, flexible optoelectronic materials, and straintronic devices. Research on local strain engineering on 2D TMDCs has been delved into fabrication techniques, electronic state variations, and quantum optical applications. This review begins by summarizing the state-of-the-art methods for introducing local strain into 2D TMDCs, followed by an exploration of the impact of local strain engineering on optical properties. The intriguing phenomena resulting from local strain, such as exciton funnelling and anti-funnelling, are also discussed. We then shift the focus to the application of locally strained 2D TMDCs as quantum emitters, with various strategies outlined for modulating the properties of TMDC-based quantum emitters. Finally, we discuss the remaining questions in this field and provide an outlook on the future of local strain engineering on 2D TMDCs.

Gravity-assisted gradient size exclusion separation of microparticles by gap-modifiable silicon nanowire arrays

The separation and detection of microparticles within complex samples pose substantial challenges due to the intricate variations in size and concentration. A strategy employing gravity-assisted gradient size exclusion principle based on controllable gap sizes on the surface of silicon nanowire arrays (SiNWAs) has been established to achieve the separation of microparticles with diverse sizes. The formation of gradient gap sizes was accomplished by meticulously investigating the impact of oxidation-reduction reactions through metal-assisted chemical etching. Particles of different sizes were initially aggregated at the accumulation base, followed by a sequential size exclusion process within the finely regulated 0.9-12.5 μm gradient-gap-sized separation region facilitated with gravity-assisted, leading to a comprehensive separation of microparticles based on their respective size differences, progressing from small to large. The effective separation of four model-sized microparticles demonstrated a separation degree of ≥2.7, purity of ≥96.1 %, RSDs of ≤4.6 %, and a separation capacity of up to 107 particles. The separation efficacy of this gradient-sized chip was verified by evaluating the more complex atmospheric particulates with varying sizes, which exhibited separation degree ranging between 2 and 10. This method offers a precise separation range, easily adjustable separation sizes, and simple operation, rendering it a versatile tool for separating complex samples.

Cationic cellulose dispersed Ag NCs/C-CNF paper-based SERS substrate with high homogeneity for creatinine and uric acid detection

To overcome the problems of easy aggregation, poor reproducibility and homogeneity of metal nanosols, a SERS substrate with good sensitivity, homogeneity and reproducibility was designed and prepared for the detection of disease markers in urine. Silver nanocubes (Ag NCs) were firstly prepared and then dispersed in cationic cellulose (C-CNF) to form a homogeneous gel, which was dropped on a filter paper to develop a substrate with good SERS activity. This substrate combines the superior SERS properties of Ag NCs with the stability of C-CNF and has a minimum detection concentration of 10-9 M for R6G. The homogeneity of this substrate was good and the RSD value was much <20 %. The SERS substrate was employed for the quantification of creatinine and uric acid, with linear ranges were 5 × 10-3-5 × 10-7 M and 10-2-10-6 M. The recoveries of creatinine and uric acid were calculated to be 98.3 % ∼ 103.12 % and 96.72 % ∼ 104.48 %, respectively, in the spike recovery experiments, with the relative standard deviations of <10 %. The experimental results show that the method can provide a scientific and reliable experimental basis for screening, condition monitoring and treatment of kidney and other diseases.

Photoluminescence modal splitting via strong coupling in hybrid Au/WS2/GaP nanoparticle-on-mirror cavities

By integrating dielectric and metallic components, hybrid nanophotonic devices present promising opportunities for manipulating nanoscale light-matter interactions. Here, we investigate hybrid nanoparticle-on-mirror optical cavities, where semiconductor WS2 monolayers are positioned between gallium phosphide (GaP) nanoantennas and a gold mirror, thereby establishing extreme confinement of optical fields. Prior to integration of the mirror, we observe an intermediate coupling regime from GaP nanoantennas covered with WS2 monolayers. Upon introduction of the mirror, enhanced interactions lead to modal splitting in the exciton photoluminescence spectra, spatially localized within the dielectric-metallic gap. Using a coupled harmonic oscillator model, we extract an average Rabi splitting energy of 22.6 meV at room temperature, at the onset of the strong coupling regime. Moreover, the characteristics of polaritonic emission are revealed by the increasing Lorentzian linewidth and energy blueshift with increasing excitation power. Our findings highlight hybrid nanophotonic structures as novel platforms for controlling light-matter coupling with atomically thin materials.

Reviewing advances in nanophotonic biosensors

Biosensing, a promising branch of exploiting nanophotonic devices, enables meticulous detection of subwavelength light, which helps to analyze and manipulate light-matter interaction. The improved sensitivity of recent high-quality nanophotonic biosensors has enabled enhanced bioanalytical precision in detection. Considering the potential of nanophotonics in biosensing, this article summarizes recent advances in fabricating nanophotonic and optical biosensors, focusing on their sensing function and capacity. We typically classify these types of biosensors into five categories: phase-driven, resonant dielectric nanostructures, plasmonic nanostructures, surface-enhanced spectroscopies, and evanescent-field, and review the importance of enhancing sensor performance and efficacy by addressing some major concerns in nanophotonic biosensing, such as overcoming the difficulties in controlling biological specimens and lowering their costs for ease of access. We also address the possibility of updating these technologies for immediate implementation and their impact on enhancing safety and health.

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

Plasmon-Free Surface-Enhanced Raman Spectroscopy Using α-Type MoO3 Semiconductor Nanorods with Strong Light Scattering in the Visible Regime

Recent developments in semiconductor-based surface-enhanced Raman scattering (SERS) have achieved numerous advancements, primarily centered on the chemical mechanism. However, the role of the electromagnetic (electromagnetic mechanism) contribution in advancing semiconductor SERS substrates is still underexplored. In this study, we developed a SERS substrate based on densely aligned α-type MoO3 (α-MoO3) semiconductor nanorods (NRs) with rectangular parallelepiped ribbon shapes with width measuring several hundred nanometers. These structural attributes strongly affect light transport in the visible range by multiple light scattering generated in narrow gaps between NRs, contributing to the improvement of SERS performance. Engineering the nanostructure and chemical composition of NRs realized high SERS sensitivity with an enhancement factor of 2 × 108 and a low detection limit of 5 × 10-9 M for rhodamine 6G (R6G) molecules, which was achieved by the stoichiometric NR sample with strong light scattering. Furthermore, it was observed that the scattering length becomes significantly shorter compared with the excitation wavelength in the visible regime, which indicates that light transport is strongly modified by mesoscopic interference related to Anderson localization. Additionally, high electric fields were found to be localized on the NR surfaces, depending on the excitation wavelength, similar to the SERS response. These optical phenomena indicate that electromagnetic excitation processes play an important role in plasmon-free SERS platforms based on α-MoO3 NRs. We postulate that our study provides important guidance for designing effective EM-based SERS-active semiconductor substrates.

From Genotype to Phenotype: Raman Spectroscopy and Machine Learning for Label-Free Single-Cell Analysis

Raman spectroscopy has made significant progress in biosensing and clinical research. Here, we describe how surface-enhanced Raman spectroscopy (SERS) assisted with machine learning (ML) can expand its capabilities to enable interpretable insights into the transcriptome, proteome, and metabolome at the single-cell level. We first review how advances in nanophotonics-including plasmonics, metamaterials, and metasurfaces-enhance Raman scattering for rapid, strong label-free spectroscopy. We then discuss ML approaches for precise and interpretable spectral analysis, including neural networks, perturbation and gradient algorithms, and transfer learning. We provide illustrative examples of single-cell Raman phenotyping using nanophotonics and ML, including bacterial antibiotic susceptibility predictions, stem cell expression profiles, cancer diagnostics, and immunotherapy efficacy and toxicity predictions. Lastly, we discuss exciting prospects for the future of single-cell Raman spectroscopy, including Raman instrumentation, self-driving laboratories, Raman data banks, and machine learning for uncovering biological insights.

Scattering/Fluorescence Dual-Mode Imaging in MnO2-Coated Silicon Nanospheres for Cancer Cell Detection

A tumor microenvironment (TME)-responsive nanoprobe composed of a fluorescent dye-decorated silicon (Si) nanosphere core and a thin MnO2 shell is proposed for simple and intelligent detection of cancer cells. The Si nanosphere core with diameters of 100-200 nm provides environment-independent Mie scattering imaging, while, simultaneously, the MnO2 shell provides the capability to switch the on/off state of the dye fluorescence reacted to the glutathione (GSH) and/or H2O2 levels in a cancer cell. Si-MnO2 core-shell nanosphere probes are fabricated in a solution-based process from crystalline Si nanosphere cores. The fluorescence switching under exposure to GSH is demonstrated, and the mechanism is discussed based on detailed optical characterizations including single-particle spectroscopy. Different types of human cells are incubated with the nanoprobes, and a proof of concept experiment is performed. From the combination of the robust scattering images and GSH- and H2O2-sensitive fluorescence images, the feasibility of cancer cell detection by the multimodal nanoprobes is demonstrated.

TiO2 Nano-Biopatterning Reveals Optimal Ligand Presentation for Cell-Matrix Adhesion Formation

Nanoscale organization of transmembrane receptors is critical for cellular functions, enabled by the nanoscale engineering of bioligand presentation. Previously, a spatial threshold of ≤60 nm for integrin binding ligands in cell-matrix adhesion is demonstrated using monoliganded gold nanoparticles. However, the ligand geometric arrangement is limited to hexagonal arrays of monoligands, while plasmonic quenching limits further investigation by fluorescence-based high-resolution imaging. Here, these limitations are overcome with dielectric TiO2 nanopatterns, eliminating fluorescence quenching, thus enabling super-resolution fluorescence microscopy on nanopatterns. By dual-color super-resolution imaging, high precision and consistency among nanopatterns, bioligands, and integrin nanoclusters are observed, validating the high quality and integrity of both nanopattern functionalization and passivation. By screening TiO2 nanodiscs with various diameters, an increase in fibroblast cell adhesion, spreading area, and Yes-associated protein (YAP) nuclear localization on 100 nm diameter compared with smaller diameters was observed. Focal adhesion kinase is identified as the regulatory signal. These findings explore the optimal ligand presentation when the minimal requirements are sufficiently fulfilled in the heterogenous extracellular matrix network of isolated binding regions with abundant ligands. Integration of high-fidelity nano-biopatterning with super-resolution imaging allows precise quantitative studies to address early signaling events in response to receptor clustering and their nanoscale organization.

Enhancement Factors: A Central Concept during 50 Years of Surface-Enhanced Raman Spectroscopy

In this Perspective, we provide an overview of the core concepts around surface-enhanced Raman spectroscopy (SERS) enhancement factors (EFs), including both theoretical and experimental considerations: EF definitions, the distinction between maximum and average EFs, EF distribution and hot-spot localization, EF measurement and its order of magnitude. We then highlight some of the current challenges in this field, focusing on a selection of topics that we feel are both topical and important: analyte-capture onto a SERS substrate, surface-enhanced resonant Raman scattering, orientation/tensorial effects, and nonradiative effects. We hope this Perspective can provide a platform to reflect on the past 50 years of SERS and its future.

Fabrication of Mie-resonant silicon nanoparticles using laser annealing for surface-enhanced fluorescence spectroscopy

Silicon nanostructures with unique Mie resonances have garnered considerable attention in the field of nanophotonics. Here, we present a simple and efficient method for the fabrication of silicon (Si) nanoparticle substrates using continuous-wave (CW) laser annealing. The resulting silicon nanoparticles exhibit Mie resonances in the visible region, and their resonant wavelengths can be precisely controlled. Notably, laser-annealed silicon nanoparticle substrates show a 60-fold enhancement in fluorescence. This tunable and fluorescence-enhancing silicon nanoparticle platform has tremendous potential for highly sensitive fluorescence sensing and biomedical imaging applications.

Coherent Electron-Vibron Interactions in Surface-Enhanced Raman Scattering (SERS)

In this Letter we identify coherent electron-vibron interactions between near-resonant and nonresonant electronic levels that contribute beyond standard optomechanical models for off-resonant or resonance surface-enhanced Raman scattering (SERS). By developing an open-system quantum model using first molecular interaction principles, we show how the Raman interference of both resonant and nonresonant contributions can provide several orders of magnitude modifications of the SERS peaks with respect to former optomechanical models and over the fluorescence backgrounds. This cooperative optomechanical mechanism allows for generating an enhancement of nonclassical photon pair correlations between Stokes and anti-Stokes photons, which can be detected by photon-counting measurements. Our results demonstrate Raman enhancements and suppressions of coherent nature that significantly impact the standard estimations of the optomechanical contribution from SERS spectra and their quantum mechanical observable effects.

Photoluminescence from FRET pairs coupled with Mie-resonant silicon nanospheres

Optically resonant nanoparticles decorated with donor-acceptor molecular pairs have been attracting attention for applications as nanoprobes in bioimaging and biosensing. We produced composite nanoparticles composed of donor-acceptor molecular pairs and silicon nanospheres (Si NSs) with diameters of 100-200 nm exhibiting Mie resonances in the visible range and studied the effect of Mie resonances on their photoluminescence properties. We showed that the photoluminescence spectra are strongly modified by Mie resonances and the spectral shape is controlled in a wide range by the Si NS size; by controlling the size, we can achieve the photoluminescence maximum from that of a donor molecule to that of an acceptor molecule almost continuously. From the photoluminescence decay properties in combination with theoretical calculations, we showed that the observed strong modification of the spectral shape is mainly due to the Purcell effect on donor and acceptor molecules, and the effect of Mie resonances on the Förster resonance energy transfer (FRET) rate is relatively small. We also showed that because of the large Purcell effect and the small FRET rate enhancement, Mie resonances decrease the FRET efficiency.

Single-emitter super-resolved imaging of radiative decay rate enhancement in dielectric gap nanoantennas

High refractive index dielectric nanoantennas strongly modify the decay rate via the Purcell effect through the design of radiative channels. Due to their dielectric nature, the field is mainly confined inside the nanostructure and in the gap, which is hard to probe with scanning probe techniques. Here we use single-molecule fluorescence lifetime imaging microscopy (smFLIM) to map the decay rate enhancement in dielectric GaP nanoantenna dimers with a median localization precision of 14 nm. We measure, in the gap of the nanoantenna, decay rates that are almost 30 times larger than on a glass substrate. By comparing experimental results with numerical simulations we show that this large enhancement is essentially radiative, contrary to the case of plasmonic nanoantennas, and therefore has great potential for applications such as quantum optics and biosensing.

Soft Epidermal Paperfluidics for Sweat Analysis by Ratiometric Raman Spectroscopy

The expanding interest in digital biomarker analysis focused on non-invasive human bodily fluids, such as sweat, highlights the pressing need for easily manufactured and highly efficient soft lab-on-skin solutions. Here, we report, for the first time, the integration of microfluidic paper-based devices (μPAD) and non-enhanced Raman-scattering-enabled optical biochemical sensing (Raman biosensing). Their integration merges the enormous benefits of μPAD, with high potential for commercialization and use in resource-limited settings, with biorecognition-element-free (but highly selective) optical Raman biosensing. The introduced thin (0.36 mm), ultra-lightweight (0.19 g), and compact footprint (3 cm2) opto-paperfluidic sweat patch is flexible, stretchable, and conforms, irritation-free, to hairless or minimally haired body regions to enable swift sweat collection. As a great advantage, this new bio-chemical sensory system excels through its absence of onboard biorecognition elements (bioreceptor-free) and omission of plasmonic nanomaterials. The proposed easy fabrication process is adaptable to mass production by following a fully sustainable and cost-effective process utilizing only basic tools by avoiding typically employed printing or laser patterning. Furthermore, efficient collection and transportation of precise sweat volumes, driven exclusively by the wicking properties of porous materials, shows high efficiency in liquid transportation and reduces biosensing latency by a factor of 5 compared to state-of-the-art epidermal microfluidics. The proposed unit enables electronic chip-free and imaging-less visual sweat loss quantification as well as optical biochemical analysis when coupled with Raman spectroscopy. We investigated the multimodal quantification of sweat urea and lactate levels ex vivo (with syntactic sweat including +30 sweat analytes on porcine skin) and achieved a linear dynamic range from 0 to 100 mmol/L during fully dynamic continuous flow characterization.

Dynamic construction of refractive index-dependent vibrations using surface plasmon-phonon polaritons

One of the fundamental hurdles in infrared spectroscopy is the failure of molecular identification when their infrared vibrational fingerprints overlap. Refractive index (RI) is another intrinsic property of molecules associated with electronic polarizability, but with limited contribution to molecular identification in mixed environments currently. Here, we investigate the coupling mode of localized surface plasmon and surface phonon polaritons for vibrational de-overlapping. The coupling mode is sensitive to the molecular refractive index, attributed to the RI-induced vibrational variations of surface phonon polaritons (SPhP) within the Reststrahlen band, referred to as RI-dependent SPhP vibrations. The RI-dependent SPhP vibrations are linked to molecular RI features. According to the deep-learning-augmented demonstration of bond-breaking-bond-making dynamic profiling in biological reaction, we substantiate that the RI-dependent SPhP vibrations effectively disentangle overlapping vibrational modes, achieving a 92% identification accuracy even for the strongly overlapping vibrational modes in the reaction. Our findings offer insights into the realm of light-matter interaction and provide a valuable toolkit for biomedicine applications.

High-aspect-ratio dielectric pillar with nanocavity backed by metal substrate in the infrared range

We investigated absorption and field enhancements of shallow nanocavities on top of high-aspect-ratio dielectric pillars in the infrared range. The structure includes a high-aspect-ratio nanopillar array of high refractive index, with nano-cavities on top of the pillars, and a metal plane at the bottom. The enhancement factor of electric field intensity reaches 3180 in the nanocavities and peak absorption reaches 99%. We also investigated the finite-size effect of the presented structure to simulate real experiments. Due to its narrow absorption bandwidth 3.5 nm, it can work as a refractive index sensor with sensitivity 297.5 nm/RIU and figure of merit 85. This paves the way to directly control light field at the nanoscales in the infrared light range. The investigated nanostructure will find applications in multifunctional photonics devices such as chips for culturing cells, refractive index sensors, biosensors of single molecule detection and nonlinear sensors.

Advances in nonlinear metasurfaces for imaging, quantum, and sensing applications

Metasurfaces, composed of artificial meta-atoms of subwavelength size, can support strong light-matter interaction based on multipolar resonances and plasmonics, hence offering the great capability of empowering nonlinear generation. Recently, owing to their ability to manipulate the amplitude and phase of the nonlinear emission in the subwavelength scale, metasurfaces have been recognized as ultra-compact, flat optical components for a vast range of applications, including nonlinear imaging, quantum light sources, and ultrasensitive sensing. This review focuses on the recent progress on nonlinear metasurfaces for those applications. The principles and advances of metasurfaces-based techniques for image generation, including image encoding, holography, and metalens, are investigated and presented. Additionally, the overview and development of spontaneous photon pair generation from metasurfaces are demonstrated and discussed, focusing on the aspects of photon pair generation rate and entanglement of photon pairs. The recent blossoming of the nonlinear metasurfaces field has triggered growing interest to explore its ability to efficiently up-convert infrared images of arbitrary objects to visible images and achieve spontaneous parametric down-conversion. This recently emerged direction holds promising potential for the next-generation technology in night-vision, quantum computing, and biosensing fields.

Photosensitizing Metasurface Empowered by Enhanced Magnetic Field of Toroidal Dipole Resonance

Photochemical reaction exploiting an excited triplet state (T1 ) of a molecule requires two steps for the excitation, i.e., electronic transition from the ground (S0 ) to singlet excited (S1 ) states and intersystem crossing to the T1  state. A dielectric metasurface coupled with photosensitizer that enables energy efficient photochemical reaction via the enhanced S0 →T1 magnetic dipole transition is developed. In the direct S0 →T1 transition, the photon energy of several hundreds of meV is saved compared to the conventional S0 → S1 →T1 transition. To maximize the magnetic field intensity on the surface, a silicon (Si) nanodisk array metasurface with toroidal dipole resonances is designed. The surface of the metasurface is functionalized with ruthenium (Ru(II)) complexes that work as a photosensitizer for singlet oxygen generation. In the coupled system, the rate of the direct S0 →T1 transition of Ru(II) complexes is 41-fold enhanced at the toroidal dipole resonance of a Si nanodisk array. The enhancement of a singlet oxygen generation rate is observed when the toroidal dipole resonance of a Si nanodisk array is matched with the direct S0 →T1 transition wavelength of Ru(II) complexes.

Turning whispering-gallery-mode responses through Fano interferences in coupled all-dielectric block-disk cavities

Here, we theoretically demonstrate a strategy for efficiently turning whispering-gallery-mode (WGM) responses of a subwavelength dielectric disk through their near-field couplings with common low-order electromagnetic resonances of a dielectric block. Both simulations and an analytical coupled oscillator model show that the couplings are Fano interferences between dark high-quality WGMs and bright modes of the block. The responses of a WGM in the coupled system are highly dependent on the strengths and the relative phases of the block modes, the coupling strength, and the decay rate of the WGM. The WGM responses of coupled systems can exceed that of the individual disk. In addition, such a configuration will also facilitate the excitation of WGMs by a normal incident plane wave in experiments. These results could enable new applications for enhancing light-matter interactions.

Dielectric metasurfaces for next-generation optical biosensing: a comparison with plasmonic sensing

In the past decades, nanophotonic biosensors have been extended from the extensively studied plasmonic platforms to dielectric metasurfaces. Instead of plasmonic resonance, dielectric metasurfaces are based on Mie resonance, and provide comparable sensitivity with superior resonance bandwidth, Q factor, and figure-of-merit. Although the plasmonic photothermal effect is beneficial in many biomedical applications, it is a fundamental limitation for biosensing. Dielectric metasurfaces solve the ohmic loss and heating problems, providing better repeatability, stability, and biocompatibility. We review the high-Q resonances based on various physical phenomena tailored by meta-atom geometric designs, and compare dielectric and plasmonic metasurfaces in refractometric, surface-enhanced, and chiral sensing for various biomedical and diagnostic applications. Departing from conventional spectral shift measurement using spectrometers, imaging-based and spectrometer-less biosensing are highlighted, including single-wavelength refractometric barcoding, surface-enhanced molecular fingerprinting, and integrated visual reporting. These unique modalities enabled by dielectric metasurfaces point to two important research directions. On the one hand, hyperspectral imaging provides massive information for smart data processing, which not only achieve better biomolecular sensing performance than conventional ensemble averaging, but also enable real-time monitoring of cellular or microbial behaviour in physiological conditions. On the other hand, a single metasurface can integrate both functions of sensing and optical output engineering, using single-wavelength or broadband light sources, which provides simple, fast, compact, and cost-effective solutions. Finally, we provide perspectives in future development on metasurface nanofabrication, functionalization, material, configuration, and integration, towards next-generation optical biosensing for ultra-sensitive, portable/wearable, lab-on-a-chip, point-of-care, multiplexed, and scalable applications.

Sub-picosecond collapse of molecular polaritons to pure molecular transition in plasmonic photoswitch-nanoantennas

Molecular polaritons are hybrid light-matter states that emerge when a molecular transition strongly interacts with photons in a resonator. At optical frequencies, this interaction unlocks a way to explore and control new chemical phenomena at the nanoscale. Achieving such control at ultrafast timescales, however, is an outstanding challenge, as it requires a deep understanding of the dynamics of the collectively coupled molecular excitation and the light modes. Here, we investigate the dynamics of collective polariton states, realized by coupling molecular photoswitches to optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an ultrafast collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation at room temperature. Through a synergistic combination of experiments and quantum mechanical modelling, we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state.

Impact of film thickness in laser-induced periodic structures on amorphous Si films

We report self-organized periodic nanostructures on amorphous silicon thin films by femtosecond laser-induced oxidation. The dependence of structural periodicity on the thickness of silicon films and the substrate materials is investigated. The results reveal that when silicon film is 200 nm, the period of self-organized nanostructures is close to the laser wavelength and is insensitive to the substrates. In contrast, when the silicon film is 50 nm, the period of nanostructures is much shorter than the laser wavelength, and is dependent on the substrates. Furthermore, we demonstrate that, for the thick silicon films, quasi-cylindrical waves dominate the formation of periodic nanostructures, while for the thin silicon films, the formation originates from slab waveguide modes. Finite-difference time-domain method-based numerical simulations support the experimental discoveries.

Recent Advances in Metaphotonic Biosensors

Metaphotonic devices, which enable light manipulation at a subwavelength scale and enhance light-matter interactions, have been emerging as a critical pillar in biosensing. Researchers have been attracted to metaphotonic biosensors, as they solve the limitations of the existing bioanalytical techniques, including the sensitivity, selectivity, and detection limit. Here, we briefly introduce types of metasurfaces utilized in various metaphotonic biomolecular sensing domains such as refractometry, surface-enhanced fluorescence, vibrational spectroscopy, and chiral sensing. Further, we list the prevalent working mechanisms of those metaphotonic bio-detection schemes. Furthermore, we summarize the recent progress in chip integration for metaphotonic biosensing to enable innovative point-of-care devices in healthcare. Finally, we discuss the impediments in metaphotonic biosensing, such as its cost effectiveness and treatment for intricate biospecimens, and present a prospect for potential directions for materializing these device strategies, significantly influencing clinical diagnostics in health and safety.

Meta-Atom Coupling Induced Chiral Hotspot in Silicon Nitride Staggered Nanorods Meta-Surface

Dielectric meta-surfaces have emerged as an effective way for fabricating chiral optical devices, and the chiral meta-surfaces are usually constituted by periodic chiral meta-atom structures. Here, we report a chiral meta-surface consisting of nonchiral silicon nitride rectangular nanorods. The chiral hotspots are generated between the staggered nanorods due to the coupling between the two nearest neighbor nanorod units. 14.6% macroscopic circular dichroism (CD) is achieved experimentally with larger area staggered nanorods. Meanwhile, we demonstrate that the wavelength tuning capability of this design from 696 to 820 nm by simply modulating the overlap length of nanorods. Our work highlights the mechanisms for CD hotspot generation without complex chiral units, which paves a novel way for future on-chip photon-spin selective devices.

Fluorophore-Decorated Mie Resonant Silicon Nanosphere for Scattering/Fluorescence Dual-Mode Imaging

Inorganic nanoparticles with multiple functions have been attracting attention as multimodal nanoprobes in bioimaging, biomolecule detection, and medical diagnosis and treatment. A drawback of conventional metallic nanoparticle-based nanoprobes is the Ohmic losses that lead to fluorescence quenching of attached molecules and local heating under light irradiation. Here, metal-free nanoprobes capable of scattering/fluorescence dual-mode imaging are developed. The nanoprobes are composed of a silicon nanosphere core having efficient Mie scattering in the visible to near infrared range and a fluorophore doped silica shell. The dark-field scattering and photoluminescence images/spectra for nanoprobes made from different size silicon nanospheres and different kinds of fluorophores are studied by single particle spectroscopy. The fluorescence spectra are strongly modified by the Mie modes of a silicon nanosphere core. By comparing scattering and fluorescence spectra and calculated Purcell factors, the fluorescence enhancement factor is quantitatively discussed. In vitro scattering/fluorescence imaging studies on human cancer cells demonstrate that the developed nanoparticles work as scattering/fluorescence dual-mode imaging nanoprobes.

Multiplexed Long-Range Electrohydrodynamic Transport and Nano-Optical Trapping with Cascaded Bowtie Photonic Crystal Nanobeams

Photonic crystal cavities with bowtie defects that combine ultrahigh Q and ultralow mode volume are theoretically studied for low-power nanoscale optical trapping. By harnessing the localized heating of the water layer near the bowtie region, combined with an applied alternating current electric field, this system provides long-range electrohydrodynamic transport of particles with average radial velocities of 30  μm/s towards the bowtie region on demand by switching the input wavelength. Once transported to a given bowtie region, synergistic interaction of optical gradient and attractive negative thermophoretic forces stably trap a 10 nm quantum dot in a potential well with a depth of 10  k_{B}T using a mW input power.

Computational Investigation of Advanced Refractive Index Sensor Using 3-Dimensional Metamaterial Based Nanoantenna Array

Photonic researchers are increasingly exploiting nanotechnology due to the development of numerous prevalent nanosized manufacturing technologies, which has enabled novel shape-optimized nanostructures to be manufactured and investigated. Hybrid nanostructures that integrate dielectric resonators with plasmonic nanostructures are also offering new opportunities. In this work, we have explored a hybrid coupled nano-structured antenna with stacked multilayer lithium tantalate (LiTaO₃) and Aluminum oxide (Al₂O₃), operating at wavelength ranging from 400 nm to 2000 nm. Here, the sensitivity response has been explored of these nano-structured hybrid arrays. It shows a strong electromagnetic confinement in the separation gap (g) of the dimers due to strong surface plasmon resonance (SPR). The influences of the structural dimensions have been investigated to optimize the sensitivity. The designed hybrid coupled nanostructure with the combination of 10 layers of gold (Au) and Lithium tantalate (LiTaO₃) or Aluminum oxide (Al₂O₃) (five layers each) having height, h₁ = h₂ = 10 nm exhibits 730 and 660 nm/RIU sensitivity, respectively. The sensitivity of the proposed hybrid nanostructure has been compared with a single metallic (only gold) elliptical paired nanostructure. Depending on these findings, we demonstrated that a roughly two-fold increase in the sensitivity (S) can be obtained by utilizing a hybrid coupled nanostructure compared to an identical nanostructure, which competes with traditional sensors of the same height, (h). Our innovative novel plasmonic hybrid nanostructures provide a framework for developing plasmonic nanostructures for use in various sensing applications.

Planar Optical Cavities Hybridized with Low-Dimensional Light-Emitting Materials

Low-dimensional light-emitting materials have been actively investigated due to their unprecedented optical and optoelectronic properties that are not observed in their bulk forms. However, the emission from low-dimensional light-emitting materials is generally weak and difficult to use in nanophotonic devices without being amplified and engineered by optical cavities. Along with studies on various planar optical cavities over the last decade, the physics of cavity-emitter interactions as well as various integration methods are investigated deeply. These integrations not only enhance the light-matter interaction of the emitters, but also provide opportunities for realizing nanophotonic devices based on the new physics allowed by low-dimensional emitters. In this review, the fundamentals, strengths and weaknesses of various planar optical resonators are first provided. Then, commonly used low-dimensional light-emitting materials such as 0D emitters (quantum dots and upconversion nanoparticles) and 2D emitters (transition-metal dichalcogenide and hexagonal boron nitride) are discussed. The integration of these emitters and cavities and the expect interplay between them are explained in the following chapters. Finally, a comprehensive discussion and outlook of nanoscale cavity-emitter integrated systems is provided.

Rapid Fabrication of Fe and Pd Thin Films as SERS-Active Substrates via Dynamic Hydrogen Bubble Template Method

Fe and Pd thin film samples have been fabricated in a rapid fashion utilizing the versatile technique of dynamic hydrogen bubble template (DHBT) method via potentiostatic electrodeposition over a copper substrate. The morphology of the samples is dendritic, with the composition being directly proportional to the deposition time. All the samples have been tested as SERS substrates for the detection of Rhodamine 6G (R6G) dye. The samples perform very well, with the best performance shown by the Pd samples. The lowest detectable R6G concentration was found to be 10^-6 M (479 μgL^-1) by one of the Pd samples with the deposition time of 180 s. The highest enhancement of signals noticed in this sample can be attributed to its morphology, which is more nanostructured compared to other samples, which is extremely conducive to the phenomenon of localized surface plasmon resonance (LSPR). Overall, these samples are cheaper, easy to prepare with a rapid fabrication method, and show appreciable SERS performance.

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Security labeling and optical information encryption enabled by laser-printed silicon Mie resonators

Fighting against the falsification of valuable items remains a crucial social-threatening challenge stimulating a never-ending search for novel anti-counterfeiting strategies. The demanding security labels must simultaneously address multiple requirements (high density of the recorded information, high protection degree, etc.) and be realized via scalable and inexpensive technologies. Here, the direct reproducible femtosecond-laser patterning of thin glass-supported amorphous (α-)Si films is proposed for optical information encryption and the scalable and highly reproducible fabrication of security labels composed of Raman-active hemispherical Si nanoparticles (NPs). Laser printing conditions allow the precise control of the diameter of the formed NPs ensuring translation of their dipolar Mie resonance position within the entire visible spectral range. Two-temperature molecular dynamics simulations clarify the origin of α-Si NP formation by rupture of the molten Si layer driven by a negative GPa-range pressure near the liquid-solid interface. Arrangement of the laser-printed Mie-resonant NP allows the creation of hidden security labels offering several easy-to-realize information encryption strategies (for example, local laser-induced post-crystallization or mixing Mie-resonant and non-resonant NPs), additional protection modalities, facile Raman mapping readout and dense information recording (up to 60 000 dots per inch) close to the optical diffraction limit. The developed fabrication strategy is simple, inexpensive, and scalable and can be realized based on cheap Earth-abundant materials and commercially-available equipment justifying its practical applicability and attractiveness for anti-counterfeit and security applications.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Recent Advances in Tunable Metasurfaces: Materials, Design, and Applications

Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc., and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

Nonlinear optical heating of all-dielectric super-cavity: efficient light-to-heat conversion through giant thermorefractive bistability

Optical heating of resonant nanostructures is one of the key issues in modern nanophotonics, being either harmful or desirable effect depending on the applications. Despite a linear regime of light-to-heat conversion being well-studied both for metal and semiconductor resonant systems is generalized as a critical coupling condition, the clear strategy to optimize optical heating upon high-intensity light irradiation is still missing. This work proposes a simple analytical model for such a problem, taking into account material properties changes caused by the heating. It allows us to derive a new general critical coupling condition for the nonlinear case, requiring a counterintuitive initial spectral mismatch between the pumping light frequency and the resonant one. Based on the suggested strategy, we develop an optimized design for efficient nonlinear optical heating, which employs a cylindrical nanoparticle supporting the quasi bound state in the continuum mode (quasi-BIC or so-called 'super-cavity mode') excited by the incident azimuthal vector beam. Our approach provides a background for various nonlinear experiments related to optical heating and bistability, where self-action of the intense laser beam can change resonant properties of the irradiated nanostructure.

Self-Assembled Au Nanoparticle Monolayers on Silicon in Two- and Three-Dimensions for Surface-Enhanced Raman Scattering Sensing

Gold nanoparticle/silicon composites are canonical substrates for sensing applications because of their geometry-dependent physicochemical properties and high sensing activity via surface-enhanced Raman spectroscopy (SERS). The self-assembly of gold nanoparticles (AuNPs) synthesized via wet-chemistry on functionalized flat silicon (Si) and vertically aligned Si nanowire (VA-SiNW) arrays is a simple and cost-effective approach to prepare such substrates. Herein, we report on the critical parameters that influence nanoparticle coverage, aggregation, and assembly sites in two- and three-dimensions to prepare substrates with homogeneous optical properties and SERS activity. We show that the degree of AuNP aggregation on flat Si depends on the silane used for the Si functionalization, while the AuNP coverage can be adjusted by the incubation time in the AuNP solution, both of which directly affect the substrate properties. In particular, we report the reproducible synthesis of nearly touching AuNP chain monolayers where the AuNPs are separated by nanoscale gaps, likely to be formed due to the capillary forces generated during the drying process. Such substrates, when used for SERS sensing, produce a uniform and large enhancement of the Raman signal due to the high density of hot spots that they provide. We also report the controlled self-assembly of AuNPs on VA-SiNW arrays, which can provide even higher Raman signal enhancement. The directed assembly of the AuNPs in specific regions of the SiNWs with a control over NP density and monolayer morphology (i.e., isolated vs nearly touching NPs) is demonstrated, together with its influence on the resulting SERS activity.

Enhancement of Molecular Coherent Anti-Stokes Raman Scattering with Silicon Nanoantennas

Surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) takes advantage of surface plasmon resonances supported on metallic nanostructures to amplify the coherent Raman response of target molecules. While these metallic antennas have found significant success in SE-CARS studies, photoinduced morphological changes to the nanoantenna under ultrafast excitation introduce significant hurdles in terms of stability and reproducilibty. These hurdles need to be overcome in order to establish SE-CARS as a reliable tool for rapid biomolecular sensing. Here, we address this challenge by performing molecular CARS measurements enhanced by nanoantennas made from high-index dielectric particles with more favorable thermal properties. We present the first experimental demonstration of enhanced molecular CARS signals observed at Si nanoantennas, which offer much improved thermal stability compared to their metallic counterparts.

Electric field enhancement by a hybrid dielectric-metal nanoantenna with a toroidal dipole contribution

Plasmonic nanocavities enable extreme light-matter interactions by pushing light down to the nanoscale. The numerical simulation is carried out systematically on the slotted Φ-shaped Si disk system with the super-dipole mode based on the analysis of the scattering strength of electric and toroidal dipoles. New blocks are introduced to the zero-field strength region of a slotted Si disk system as a function of the field enhancement factors. The far-field scattering characteristics and near-field electromagnetic field distributions are investigated by a multipole decomposition analysis to elucidate the intrinsic causes of the field enhancement. In the hybrid metal-dielectric nanoantenna, the Φ-shaped Si structure is prepared by superimposing Au nanoantennas for further field enhancement. In addition, the effects of the placement of an electric dipole emitter on the Purcell factor are derived. The geometric volume of the system is increased, and the electric field strength is improved, leading to an electric field increase of ∼30. Coupling between the super-dipole mode of the dielectric nanostructure and plasmonic modes of the metallic nanoantenna produces an enhancement as large as 16 times. Our results reveal a greatly enhanced super-dipole mode by electromagnetic coupling in composite structures, which will play a significant role in enhanced nonlinear photonics, near-field enhancement spectroscopy, and strong photon-exciton coupling.

Recent advances in non-plasmonic surface-enhanced Raman spectroscopy nanostructures for biomedical applications

Surface-enhanced Raman spectroscopy (SERS) is an emerging powerful vibrational technique offering unprecedented opportunities in biomedical science for the sensitive detection of biomarkers and the imaging and tracking of biological samples. Conventional SERS detection is based on the use of plasmonic substrates (e.g., Au and Ag nanostructures), which exhibit very high enhancement factors (EF = 1010 -1011 ) but suffers from serious limitations, including light-induced local heating effect due to ohmic loss and expensive price. These drawbacks may limit detection accuracy and large-scaled practical applications. In this review, we focus on alternative approaches based on plasmon-free SERS detection on low-cost nanostructures, such as carbons, oxides, chalcogenides, polymers, silicons, and so forth. The mechanism of non-plasmonic SERS detection has been attributed to interfacial charge transfer between the substrate and the adsorbed molecules, with no photothermal side-effects but usually less EF compared with plasmonic nanostructures. The strategies to improve Raman signal detection, through the tailoring of substrate composition, structure, and surface chemistry, is reviewed and discussed. The biomedical applications, for example, SERS cell characterization, biosensing, and bioimaging are also presented, highlighting the importance of substrate surface functionalization to achieve sensitive, accurate analysis, and excellent biocompatibility. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > Biosensing Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Surface-enhanced coherent anti-Stokes Raman scattering of molecules near metal-dielectric nanojunctions

We discuss an experimental configuration consisting of {Au film}-molecule-{Au particle} or {Au film}-molecule-{Si particle} nanojunctions for performing wide-field surface-enhanced CARS (SE-CARS) measurements in a reproducible and controllable manner. While the allowable illumination dosage in the {Au film}-molecule-{Au particle} case is limited by the strong two-photon background from the gold, we successfully generate a detectable coherent Raman response from a molecular monolayer using the lowest reported average power densities to-date. With a vision to minimize the two-photon background and the intrinsic losses observed in all-metal plasmonic systems, we examine the possibility of using high-index dielectric particles on top of a thin metal film to generate strong nanoscopic hotspots. We demonstrate repeatable SE-CARS measurements at the {Au film}-molecule-{Si particle} heterojunction, underlining the usability of this experimental geometry. This work paves the way for the development of next-generation of chemical and biomolecular sensing assays that can minimize some of the major drawbacks encountered in fragile and lossy all-metal plasmonic systems.

Widefield SERS for High-Throughput Nanoparticle Screening

Surface-enhanced Raman scattering (SERS) imaging is a powerful technology with unprecedent potential for ultrasensitive chemical analysis. Point-by-point scanning and often excessively long spectral acquisition-times hamper the broad exploitation of the full analytical potential of SERS. Here, we introduce large-scale SERS particle screening (LSSPS), a multiplexed widefield screening approach to particle characterization, which is 500-1000 times faster than typical confocal Raman implementations. Beyond its higher throughput, LSSPS simultaneously quantifies both the sample's Raman and Rayleigh scattering to directly quantify the fraction of SERS-active particles which allows for an unprecedented correlation of SERS activity with particle size. .

Design and Optimization of All-Dielectric Fluorescence Enhancing Metasurfaces: Towards Advanced Metasurface-Assisted Optrodes

The need for miniaturized biological sensors which can be easily integrated into medical needles and catheters for in vivo liquid biopsies with ever-increasing performances has stimulated the interest of researchers in lab-on-fiber (LOF) technology. LOF devices arise from the integration of functional materials at the nanoscale on the tip of optical fibers, thus endowing a simple optical fiber with advanced functionalities and enabling the realization of high-performance LOF biological sensors. Consequently, in 2017, we demonstrated the first optical fiber meta-tip (OFMT), consisting of the integration of plasmonic metasurfaces (MSs) on the optical fiber end-face which represented a major breakthrough along the LOF technology roadmap. Successively, we demonstrated that label-free biological sensors based on the plasmonic OFMT are able to largely overwhelm the performance of a standard plasmonic LOF sensor, in view of the extraordinary light manipulation capabilities of plasmonic array exploiting phase gradients. To further improve the overall sensitivity, a labelled sensing strategy is here suggested. To this end, we envision the possibility to realize a novel class of labelled LOF optrodes based on OFMT, where an all-dielectric MS, designed to enhance the fluorescence emission by a labelled target molecule, is integrated on the end-face of a multimode fiber (MMF). We present a numerical environment to compute the fluorescence enhancement factor collected by the MMF, when on its tip a Silicon MS is laid, consisting of an array of cylindrical nanoantennas, or of dimers or trimers of cylindrical nanoantennas. According to the numerical results, a suitable design of the dielectric MS allows for a fluorescence enhancement up to three orders of magnitudes. Moreover, a feasibility study is carried out to verify the possibility to fabricate the designed MSs on the termination of multimode optical fibers using electron beam lithography followed by reactive ion etching. Finally, we analyze a real application scenario in the field of biosensing and evaluate the degradation in the fluorescence enhancement performances, taking into account the experimental conditions. The present work, thus, provides the main guidelines for the design and development of advanced LOF devices based on the fluorescence enhancement for labelled biosensing applications.

Flexible Silicon Dimer Nanocavity with Electric and Magnetic Enhancement

High-index dielectrics have recently been regarded as promising building blocks in nanophotonics owing to optical electric and magnetic Mie resonances. In particular, silicon is gaining great interest as the backbone of modern technology. Here, silicon dimer nanocavities with different sizes of silicon nanospheres were constructed using a probe nanomanipulation method and interacted with a few-layered R6G membrane to investigate the enhancement of electric and magnetic mode coupling. The evidence of the enhancement of fluorescence and slightly prolonged lifetime of R6G indicated the existence of nanocavities. In addition, the simulated electric and magnetic field distributions and decomposed mode of nanocavity were used to analyze the contribution of electric and magnetic modes to the R6G enhanced fluorescence. Such silicon dimer is a flexible nanocavity with electric and magnetic mode enhancement and has promising applications in sensing and all-dielectric metamaterials or nanophotonic devices.

Deep sub-wavelength localization of light and sound in dielectric resonators

Optomechanical crystals provide coupling between phonons and photons by confining them to commensurate wavelength-scale dimensions. We present a new concept for designing optomechanical crystals capable of achieving unprecedented coupling rates by confining optical and mechanical waves to deep sub-wavelength dimensions. Our design is based on a dielectric bowtie unit cell with an effective optical/mechanical mode volume of 7.6 × 10^-3(λ/n_Si)³/1.2×10^-3 λ _mech ³. We present results from numerical modeling, indicating a single-photon optomechanical coupling of 2.2 MHz with experimentally viable parameters. Monte Carlo simulations are used to demonstrate the design's robustness against fabrication disorder.

The Current State of Optical Sensors in Medical Wearables

Optical sensors play an increasingly important role in the development of medical diagnostic devices. They can be very widely used to measure the physiology of the human body. Optical methods include PPG, radiation, biochemical, and optical fiber sensors. Optical sensors offer excellent metrological properties, immunity to electromagnetic interference, electrical safety, simple miniaturization, the ability to capture volumes of nanometers, and non-invasive examination. In addition, they are cheap and resistant to water and corrosion. The use of optical sensors can bring better methods of continuous diagnostics in the comfort of the home and the development of telemedicine in the 21st century. This article offers a large overview of optical wearable methods and their modern use with an insight into the future years of technology in this field.